Cancer is second only to heart disease as a cause of death, accounting for 22% of all deaths (Fraumeni J. et al. Epidemiology of cancer. In: Cancer—principles and practice of oncology, DeVita V., et al., (eds.) pp. 150, Lippincott J. R. Co., Philadelphia, 1993. Colon cancer, melanoma and breast cancer are three particularly problematic types of cancer.
Melanomas are aggressive, frequently metastatic tumors derived from either melanocytes or melanocyte-related nevus cells (“Cellular and Molecular Immunology” (1991) (eds) Abbas A. K., W.B. Saunders Company, Philadelphia: pages 340-341) which make up approximately 3% of all skin cancers. Of particular concern is the current worldwide increase in melanoma which is unsurpassed by any other neoplasm with the exception of lung cancer in women (“Cellular and Molecular Immunology” (1991) (eds) Abbas, A. K., et al., W.B. Saunders Company Philadelphia pages: 340-342; Kirkwood and Agarwala (1993) Principles and Practice of Oncology 7:1-16). The aggressiveness of melanoma is such that even when melanoma is apparently localized to the skin, up to 30% of the patients will develop systemic metastasis and the majority will die.
Breast cancer is a significant health problem for women in the United States and throughout the world. Although advances have been made in detection and treatment of the disease, breast cancer remains the second leading cause of cancer-related deaths in women, affecting more than 180,000 women in the United States each year. For women in North America, the life-time odds of getting breast cancer are now one in eight.
Colon cancer is the second most frequently diagnosed malignancy in the United States, as well as the second most common cause of cancer death. About 100,000 new cases of colon cancer are diagnosed yearly, with about 50,000 deaths. The five-year survival rate for patients with colorectal cancer detected in an early localized stage is 92%; unfortunately, only 37% of colorectal cancer is diagnosed at this stage. The survival rate drops to 64% if the cancer is allowed to spread to adjacent organs or lymph nodes, and to 7% in patients with distant metastases. Recurrence following surgery (the most common form of therapy) is a major problem and is often the ultimate cause of death. In spite of considerable research into therapies for the disease, colon cancer remains difficult to diagnose and treat.
While metastatic primary tumors can in many cases be surgically removed, metastases, such as disseminated micrometastases, can be difficult or impossible to locate and/or reach and thus surgical removal of such metastases is usually not an option. Therefore, prevention of metastasis is necessary to improve the prognosis of cancer patients.
In order to treat cancer effectively, efficient removal of the primary tumor mass, prevention of secondary tumor growth, and eradication of metastatic cells must be achieved. Surgical excision of tumors is the most widely employed therapeutic modality for the treatment of cancer, in which the primary goal is the complete eradication of local and regional tumor. This involves removal of adequate margins of normal tissue surrounding the tumor, and radical wide excision in order to prevent local recurrence. However, despite major advances in the surgical pre- and postoperative care of patients, surgical treatment of malignant neoplasms remains highly limited (Eilber, F. R., Principles of cancer surgery. In: Cancer Treatment, Haskell C M, (ed.) 5th ed., pp. 47, W.B. Saunders Co. Philadelphia, 2001). Surgical techniques are effective only in the area of the primary tumor or regional lymphatics and do not affect neoplasms located outside the operative field. Furthermore, due to anatomic location, many tumors cannot be treated by surgical resection because removal of an adequate margin of normal tissue cannot be achieved. Also, surgical treatment is often not an option for tumors intimately involving major blood vessels or essential organs. As well, many patient present problematic medical histories, such as cerebrovascular or cardiovascular accidents, or uncontrolled diabetes, rendering them poor surgical candidates because of their high postoperative mortality rate. Also, in many cases, tumor excision can not be performed without causing unacceptable levels of impairment of physiologic functions or cosmetic damage.
Chemotherapy alone or in combination with surgery is commonly the most efficient anti-cancer remedy (Haskell, C. M., Principles of cancer chemotherapy. In: Cancer Treatment (ed.) 5th ed. pp. 62-86, W.B. Saunders Co. Philadelphia, 2001). However, chemotherapeutic agents often cause severe and unacceptable side-effects, such as bone marrow and lymphoid organ damage resulting in immunosuppression, thereby rendering subjects highly vulnerable to lethal opportunistic infections, as well as various other types of organ toxicities. Thus, the use of cytotoxic drugs is limited only to tolerated doses. One way to reduce minimal therapeutic doses of chemotherapeutic agents would be to enhance the efficiency of uptake of chemotherapeutic drugs into cancer cells.
During the last two decades, various techniques based on biological, chemical and physical processes have been developed for facilitating incorporation of macromolecules into cells. Methods for intracellular delivery of exogenous substances based on biological phenomena have employed molecules controlling the activity of specific membrane channels in various cell types (Heppel and Weisman, 1985. J Membr Biol. 86:189), pore-forming toxins (Ahnert-Higler et al., 1989. Methods Cell Biology 31:63) and liposome-endocytosis mediated delivery of compounds (Friend et al., 1996. Biophys Acta 1278:41). Some permeabilization methods are based on chemical modification of cell membranes by various substances, most commonly via the use of detergents as permeabilizing agents.
Electroporation, also known as electropermeabilization or electroinjection, is the permeabilization of cell membranes as a consequence of the application of certain short and intense electric fields across the cell membrane, the cells or the tissues. This may be 50-200 μs pulses of high-strength electric fields in the range of 500-5000 V/cm. Typically, a sequence of eight 100 μs pulses of approximately 1000 V/cm is applied.
The permeabilization can be temporary (reversible electroporation) or permanent (irreversible electroporation) as a function of the electrical field magnitude and duration, and the number of pulses.